Take an electrical insulator tube, fit a flat cyllindrical conducting electrode on one end, fill the tube with powder (tap it if you may) to a certain height, and put another electrode into the other side of powder stack. Calculate ressitance. Vary the powder bed height and calculate resitance again for multiple times. Thus find out resistance without contribution from contact resistance. You know tube inner dia and length variation, as well as resistivity variation, so calculate resistivity, then find out conductivity.